Polymer compositions useful for molding applications, specifically the manufacture of caps and closures for bottles are well known. Screw closures for example, are typically made from polypropylene (PP) in order to achieve the necessary cap strength, however, an inner liner composed of a soft polymer is required to provide necessary seal properties. The soft inner liner can be made from ethylene/vinyl acetate (EVA), polyvinyl chloride (PVC), butyl rubber or other suitable material. The two-part cap is costly, and single part constructions are preferred to reduce cost.
Accordingly, one-piece closures, such as screw caps have more recently been made from polyethylene resins. The use of high density resin is required if the closures are to have sufficient stiffness, while broader molecular weight distributions are desirable to impart good flow properties and to improve environmental stress crack resistance (ESCR).
Polyethylene blends produced with conventional Ziegler-Natta or Phillips type catalysts systems can be made having suitably high density and ESCR properties, see for example, WO 00/71615 and U.S. Pat. No. 5,981,664. However, the use of conventional catalyst systems typically produces significant amounts of low molecular weight polymer chains having high comonomer contents, which results in resins having non-ideal organoleptic properties.
Examples of high density multimodal polyethylene blends made using conventional catalyst systems for the manufacture of caps or closures are taught in US 2005/0004315A1; US 2005/0267249A1; WO 2006/048254, WO 2006/048253 and WO 2007/060007.
In contrast to traditional catalysts, the use of so called single site catalysts (such as “metallocene” and “constrained geometry” catalysts) provides resin having lower catalyst residues and improved organoleptic properties as taught by U.S. Pat. Nos. 6,806,338 and 7,022,770. The disclosed resins are suitable for use in molded articles.
Despite the progress made in the field, there remains a need for new polyethylene resins having an improved balance of toughness, ESCR, processability, and organoleptic properties for use in the manufacture of caps and closures. We have now found that use of a single site catalyst system in a dual reactor solution process, provides bimodal polyethylene compositions of specific molecular weight distributions having good and balanced rheological and mechanical properties. The polyethylene compositions provided also have good organoleptic properties and are especially well suited to the manufacture of closures for bottles.